Lipoxygenase activity in tropical fish: Changes during surimi processing and some biochemical characterization in lizardfish (Saurida tumbil).

The lipoxygenase (LOX) activity of major tropical fish used for surimi production, including threadfin bream (TB), lizardfish (LZ), and goatfish (GF), was measured in the gills, skin, and muscle. The highest LOX activity was observed in the LZ samples (p < 0.05), with the gills exhibiting the greatest activity at 376.56 U/mg (p < 0.05). The highest peroxide value was detected in the TB samples, particularly in the gills (p < 0.05). Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) were the main polyunsaturated fatty acids in all the tissues and surimi. The total lipid and DHA contents of washed mince reduced considerably after the screw press process. Although LOX activity decreased during surimi production, a residual activity of 21.33 U/g was observed in the finished surimi. LOX was partially purified and characterized from LZ gills. The purification was conducted using two successive chromatographic steps, Sephacryl S-200 and diethylaminoethyl (DEAE)-sepharose, resulting in a 3.52% yield and a 22.43-fold increase in purity. The optimum activity was found at 25°C and pH 7.5, with pH stability between 6.0 and 8.5. The relatively high thermal stability at 4°C-10°C suggested that LOX might contribute to fish lipid oxidation during cold storage. The enzyme was thermally inactivated at 60°C. The preferred substrate was EPA. LOX from the LZ gills was inhibited by 1 mM ethylenediaminetetraacetic acid and activated by 1 mM Fe²⁺, Na⁺, and Ca²⁺. PRACTICAL APPLICATION: Elucidating lipoxygenase activity and lipid oxidation in various tropical fish tissues, as well as understanding the characteristics of LOX, can help take appropriate postharvest actions to afford high-quality surimi.